I’ve found it to be okay. The plugin system is nice, but the lack of local file sync on iOS is quite bothersome, since I use a self-hosted sync instead.
It basically makes the iOS app unusable for me, and it would be a deal breaker if Obsidian didn’t just use markdown documents you can edit with a text editor.
Admittedly, I migrated from Apple Notes and raw text documents, so there isn’t much by way of competition.
I have tried a bit of the others, like abusing LaTeX for it, which worked okay, but had a few flaws, like when linking other files, and Trillium, which is interesting, but also uses an SQL database (without mobile support, which didn’t work for me).
Turtl was interesting, but when I used it, very beta.
Evernote was okay, but also suffers from the proprietary format. I dropped them after they reduced the device support down to 2/3, so no idea what it’s like now.
Onenote was nice with the pen support (only Apple and Samsung notes otherwise have that, as far as I’m aware), but I found it to be quite heavy, and a little unwieldy (in addition to being proprietary).
Onenote also has a strange quirk where it will gradually accumulate copies, so big notes will cause it to grow over time until it starts eating up huge amounts of space on your computer.